This invention relates to apparatus for cutting a web of sheet material, and particularly a moving web in a processing line which includes a continuous winder.
The invention has special application to installations typified by the continuous winder shown in Harmon et at U.S. Pat. No. 3,794,255 issued Feb. 26, 1974 to the assignee of this application. The knife arrangement shown in that patent has proved to be highly successful in the handling of sheet materials of the types listed therein, namely cardboard, plastics and like materials which are relatively thick or otherwise resistant to conventional cutting knives, but it has not been satisfactory for use with webs of materials which have a high tendency to elongation under stress and/or a low coefficient of friction such for example as woven and non-woven plastic fiber or filament webs.
The problem which arises in the cutting of such webs derives from the basic fact that with any knife which relies for its cutting action on passage through an incompletely supported web, as distinguished from a guillotine type knife which cuts directly against an anvil, the tension in the web is an essential factor in the cutting action. In other words, the tension in that portion of the web which opposes the knife must be adequate to keep the web sufficiently taut for passage of the knife therethrough, rather than merely stretching the web without penetrating it.
This problem is especially acute in connection with webs which have such a high tendency to elongation under stress that they must be handled in a processing line and rewound under very low tension to prevent permanent elongation in the wound roll. A typical example of such a web is a woven plastic of such high tendency to stretching that it should not be handled at tensions exceeding four ounces per linear inch of width, but which cannot be cut successfully across an unsupported run at a tension less than ten pounds, and which may elongate as much as 50% or more in developing ten pounds of tension. If it is attempted to cut such a web with the knife arrangement of U.S. Pat. No. 3,794,255, this required elongation will occur between the knife and the winding roll causing excessive knife travel. In addition, some web materials, depending upon their coefficient of friction, will skid against the driving drum before sufficient tension can be developed to cut the web.